1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal processing method.
2. Description of the Related Art
For example, an amplitude modulation method and a phase modulation method are well known as digital modulation methods. The amplitude modulation method is also referred to as an ASK (Amplitude Shift Keying) method, i.e., a type of a digital modulation method that changes an amplitude of a carrier wave in accordance with transmission data. Another known example of the amplitude modulation method is an OSK (On-Off Shift Keying) method that associates data with either a presence or non-presence of an amplitude. On the other hand, the phase modulation method is also referred to as a PSK (Phase Shift Keying) method, i.e., a type of a digital modulation method that changes a phase of a carrier wave in accordance with transmission data. Exemplary types of the phase modulation method include a BPSK (Binary Phase Shift Keying) method, a QPSK (Quadrature Phase Shift Keying) method, and the like.
When data are transmitted by the phase modulation method, a receiver reproduces a carrier wave by, e.g., a synchronous demodulation method, and measures a phase difference between the carrier wave and the received signal, thus demodulating the data. Japanese Patent Application Laid-Open No. 2008-294730 discloses a data demodulation method of the phase demodulation method, and more specifically a technique for generating sampling data using a sampling clock having the same frequency as a carrier wave and detecting a correct phase range of the received signal to be sampled. In this technique, the sampling clock having the same frequency as the carrier wave is used to determine the data and the phase. Therefore, this technique alleviates the requirement of the operational rate, compared with a method in related art for processing data using a sampling frequency several times higher than a carrier wave. As a result, this technique enables efficient transmission of data using a carrier wave having a higher frequency.